CODEWITHSUNDEEP
haskellfrpelm
Kira Ghandhi
Kira Ghandhi

Reputation: 95

How do I merge signals in Helm?

I am working on a project using Helm, which is based on the Elm language.

I need to trigger an event based on which signal out of a pair of signals arrives first. In Elm, I would use the merge function, but I cannot find an equivalent in Helm. The closest I see is combine (in the signal library), which doesn't appear to do what I want. It seems that combine just takes a list of signals and makes them directly into a signal of lists, which is not quite what I'm looking for.

EDIT: specifically I am looking for a function with signature Signal a -> Signal a -> Signal a that takes the first signal to fire and discards the second.

What's the best way to accomplish this in Helm?

Upvotes: 4

Views: 126

Answers (2)

Apanatshka
Apanatshka

Reputation: 5958

I think the best way would be finding out how Helm's Signals are implemented in terms of Elerea's stuff, and implementing merge in those more primitive terms. I'm afraid I can't help you there, because I don't know Helm well and I don't know Elerea at all.

I can help you with a hacky solution based solely on Helm. You can use timestamp to distinguish different events from zipped signals: 

merge sigL sigR =
  let
    tsMerge (t1,v1) (t2,v2) =
        if t1 >= t2
          then v1
          else v2
  in
    tsMerge <~ timestamp sigL ~~ timestamp sigR

Upvotes: 2

Doug McClean
Doug McClean

Reputation: 14485

You can combine things using the Applicative instance. For example liftA2 (,) will have type Signal a -> Signal b -> Signal (a,b).

(I'm not 100% sure if this is your question, though. If your question is about merging events like Event a -> Event a -> Event a, I'm not sure helm has features like that. I can only find the documentation for behavior-like Signals that have continuous semantics.)

Upvotes: 2

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